Influence of chemical structure on physicochemical properties and thermal decomposition of the fully bio-based poly(propylene succinate-co-butylene succinate)s
Introduction
One from the most developing materials, which have a huge impact on everyday life, are polyurethanes. Due to their broad spectrum of properties they provide for quality of toys, furniture, containers, clothes, but also have importance in medicine and automotive industry. Recently, there are available chemical compounds derived from renewable resources which allowed to synthesize green polyurethane materials [1].
Polyols constitute one of the most important components in the polyurethane synthesis. Polyester polyols constitute 18 mas.% of all produced polyols for polyurethanes [2]. Polyurethane materials based on polyester polyols are usually characterized by higher mechanical, thermal and organic solvent resistance compared to polyether polyol-based PUs. Moreover, poly (ester-urethane)s are lower resistance on hydrolysis, so they are more liable on biodegradation in comparison with polyether polyol-based PUs [3]. The chemical structure of polyol determined the morphology, mechanical and thermal properties of PUs. The highest impact on above-mentioned properties, the polyol chemical structure exert in the case of thermoplastic polyurethane elastomers TPU. Moreover, the molecular mass distribution influences the industrial processes of TPU production. It is difficult to obtain a product of step-by-step polymerization method which will be characterized by designed chemical composition and narrow polydispersity [4]. Therefore, it is important to analyze the chemical structure of monomers before industrial processes. Moreover, one from the most responsive to the macromolecular structure are thermal transition temperatures and thermal stability of both, pure polyols and prepared polyurethanes. It was verified that the increasing molecular weight of polyol leads to the polyurethanes with enhanced mechanical properties and thermal stability [5]. Moreover, with increasing molecular weight polyols characterized by the increasing melting point and decreasing glass transition temperatures [6].
Sobkowicz et al. [7] investigated thermal and mechanical properties of poly (butylene succinate-co-hexamethylene succinate) copolyesters prepared with using a various molar ratio of the used glycols. All prepared materials characterized the average molecular weight in the range from 36 000 to 53 000 g/mol with polydispersity between 1.1 and 1.5. The results of the thermal analysis indicate that with increasing 1,6-hexanediol glycol content at the copolyesters macromolecular structure the increase at the thermal decomposition temperature was visible. The melting temperature for all prepared copolyesters decreased with increasing HDO content at the macromolecular structure. After exceeding the value of molar ratio BDO: HDO at 5:5, the melting temperature increases with growing HDO content. Mechanical properties measurements verified that the highest values of tensile strength, yield stress, and Young's modulus characterized poly (propylene succinate) and with an increasing amount of 1,6-hexanediol at the copolyesters, the value of this parameters decreases.
Papageorgiou and co-workers [8] investigated poly (butylene furanoate), poly (butylene succinate) and poly (butylene succinate-co-furanoate) which were characterized by number average molecular weight at 12 700 g/mol for PBSF and 15 200 g/mol for PBS with polydispersity 2.12 and 2.03, respectively. Authors didn't quote the value of Mn for PBF polyester. They confirmed the values of glass transition temperatures at −36 °C and 34 °C for PBS and PBF, respectively. The use of carboxylic acids mixture allowed obtaining copolyester PBSF characterized by Tg at −29 °C. Moreover, PBF polyester revealed crystallization temperature at 91 °C, when copolyester PBSF at 9 °C, which made it more favorable to use during industrial processes. PBSF copolyester featured also lower melting temperature than PBS and PBF polyesters, ca. 100, 110 and 170 °C, respectively. Authors investigated also thermal stability of all polyesters. The results indicate that the lower value of the temperature of the beginning of thermal decomposition characterized polyester PBF (325 °C) when the highest – PBS (353 °C). Copolyester PBSF revealed this temperature at 331 °C.
Loos et al. [9] investigated bio-based polyesters synthesized with the use of 2,5-bis(hydroxymethyl)furan and diacids ethyl esters of such acids as succinic, glutaric, adipic, suberic, sebacic and dodecanedioic. The results of average molecular weight measurements indicate that all polyesters characterized by Mn at ca. 2000 g/mol. Moreover, authors confirmed that with an increasing amount of methylene units in the dicarboxylic segments, increases the degree of crystallinity of polyesters. Moreover, with an increasing amount of methylene units in the dicarboxylic segments, synthesized polyesters characterized by higher values of thermal stability and melting points, which ranged from 253 to 300 °C, and from 55 to 86 °C, respectively.
In this work, six linear bio-based polyols dedicated for thermoplastic polyurethane elastomers were synthesized. Two polyesters and four copolyesters were produced with the use of succinic acid, 1,3-propanediol, and 1,4-butanediol, all with a natural origin. Due to the requirements concerning polyester polyols for thermoplastic polyurethane elastomers, the prepared bio-based polyester polyols have to be characterized by the low value of an acid number and hydroxyl number, an average molecular weight in the range from 1000 to 4000 g/mol, functionality equaled 2, and low water content. The chemical structure was analysis by FTIR and 1H NMR spectroscopy. The resulted spectra confirmed the occurrence of ester groups at the macromolecular structure and shown differences in the chemical structure between polyesters and copolyesters. Moreover, the majority of probable chemical structures of macromolecules formed during synthesis were proposed based on the results of MALDI-ToF MS measurements. Thermal stability of synthesized bio-based polyester and copolyester polyols were investigated for studying the impact of the chemical structure on the thermal degradation temperature. Furthermore, the thermal decomposition mechanisms were determined with the use of DSC-TG/QMS coupled method.
Section snippets
Materials
The main components used in this study constituted dicarboxylic acid which was succinic acid (SA), and two glycols: 1,3-propanediol (PDO) and 1,4-butanediol (BDO). All from the mentioned substrates were the natural origin. Succinic acid – Biosuccinium, was kindly supplied by Reverdia (Netherlands). SA characterized as a white powder with purity in the range 98–100% and the molecular weight at 118.09 g/mol. PDO – Susterra Propanediol, was obtained from DuPont Tate&Lyle Corporation Bio Products
Bio-based polyester and copolyester polyols characterization
Table 1 shown the synthesis reaction conditions and the results of the acid and hydroxyl numbers of bio-based poly (propylene succinate), bio-based poly (butylene succinate) and bio-based poly (propylene succinate-co-butylene succinate)s. Samples named PPS and PBS represent polyols synthesized with the use of one type of glycols, PDO and BDO, respectively. Specimens called SPB 1, SPB 2, SPB 3 and SPB 4 are polyols synthesized using glycols mixture with different PDO: BDO molar ratios. All
Conclusion
In this work, two polyesters and four co-polyesters were synthesized with the use of renewable resources. All prepared bio-based materials were dedicated as polyols for thermoplastic polyurethane elastomers. The used synthesis conditions allowed to obtain polyols characterized by primary properties required for TPUs synthesis such as an acid number at ca. 1 mg KOH/g, a hydroxyl number in the range from 40 to 100 mg KOH/g, an average number molecular weight ca. 2000 g/mol and water content lower
CRediT authorship contribution statement
Paulina Parcheta: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration. Janusz Datta: Visualization, Project administration.
Declaration of competing interest
With the submission, I would like to undertake the responsibility that the authors of the manuscript do not have any conflicts of interest.
Acknowledgments
The sincere acknowledgments are directed for the DuPont Tate&Lyle Corporation (USA) and BASF (Germany) for supplying the bio-based 1.3-propanediol and bio-based 1.4-butanediol, respectively, samples used in this study. The authors gratefully acknowledge also receiving the samples of bio-based succinic acid employed in this study from Myriant (USA), BioAmber (USA) and Reverdia (Netherlands).
Thanks are also due to Mrs. Dr. Iwona Koltsov from the Laboratory of Nanostructures for Photonic and
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